An exhaust-gas turbocharger and a method for improving its efficiency are described in German Patent Application No. DE 31 01 131. The exhaust-gas turbocharger has an overrun control valve (safety valve) and an integrated bypass duct, which opens out into the exhaust duct, in back of the turbine part. The discharged bypass mass stream is expanded in an injector nozzle for generating kinetic energy, the expanded bypass mass stream being introduced into the mass stream of exhaust gas in such a manner, that the back pressure in back of the turbine part decreases. The discharge of the bypass duct into the exhaust duct in the form of an injector nozzle is designed so that it introduces the bypass mass stream into the mass stream of exhaust gas of the turbine in parallel or at an acute angle. According to German Patent Application No. DE 31 01 131, the angle formed between the bypass mass stream and the mass stream of exhaust gas is less than 30°.
The power limit of a charging device (supercharger), such as an exhaust-gas turbocharger, is expanded, for example, using two-stage controlled supercharging, as is known from the Bosch Automotive Handbook, 23rd Edition, Vieweg, 1999, pp. 445-446. In two-stage controlled supercharging, two exhaust-gas turbochargers of different size are connected in series. The mass stream of exhaust gas initially flows into an exhaust manifold. From here, the mass stream of exhaust gas is expanded via a high-pressure turbine. In the case of large amounts of exhaust gas, as occur at high engine speeds, a part of the mass stream of exhaust gas may be rerouted around the high-pressure turbine via a bypass. The entire mass stream of exhaust gas is then utilized by a low-pressure turbine downstream from the high-pressure turbine. The mass stream of fresh air drawn in is initially precompressed by a low-pressure stage and subsequently compressed further in the high-pressure stage. The mass stream of fresh air is ideally intercooled between the low-pressure stage and the high-pressure stage.
At approximately 50% to 60% of the rated speed, most of the exhaust gas is directed around the high-pressure turbine via a bypass. Since the entrance to the high-pressure turbine is not closed, exhaust gas continues to flow through the high-pressure turbine. The high-pressure compressor is simultaneously bypassed via a charge-air line, in which a nonreturn valve is provided, in order to prevent charge air from flowing back through the charge-air line during operation of the high-pressure compressor.
Two-stage supercharging in a charging device is generally accomplished by two exhaust-gas turbochargers connected in series. This allows two-stage expansion via the two turbine parts of the two exhaust-gas turbochargers to be achieved, like two-stage compression on the compressor side of the two exhaust-gas turbochargers interconnected in series. The disadvantages of uncontrolled, two-stage supercharging are prevented by control elements for bypassing the high-pressure turbine and the high-pressure compressor.
The above-described, two-stage supercharging is becoming increasingly important in the evolution of charging systems for internal combustion engines. Because of the fact that in the course of the two-stage controlled supercharging of an internal combustion engine, a part of the exhaust gases is conveyed directly from the internal combustion and the high-pressure compressor to the low-pressure compressor, it is exceedingly important that this exhaust-gas stream be conveyed in an extremely low-loss manner. German Patent Application No. DE 31 01 131 does show a bypass-duct design approach, but it describes circumference-angle requirements, which only produce the desired jet-pump effect to a limited extent.